Delayed coking is a well-established process in the industry, which produces more desirable lighter distillates along with petroleum coke from petroleum residuum (bottoms from atmospheric and vacuum distillation of crude oil). Delayed coking has become more important in recent years due to the declining demand of fuel oil, deteriorating crude quality and also rising crude prices. In most of the refineries, the process is considered as a residue disposal unit, but the same process has also been found to be an excellent route for the production of premium grade or needle coke from certain selected feedstocks.
Needle coke, named for its needle-like structure, is the highest value petroleum coke used for manufacturing graphite electrode for arc furnaces. A large-sized artificial graphite electrode requires lower coefficient of thermal expansion as well as electrical resistivity, high density and a high level of physical strength, in order to cope with the operation of steel-making electric arc furnace. The material, consequently, should contain a large quantity of the so-called needle coke having needle-like crystalline structure of easily and readily graphitizable nature.
A number of factors, such as sulfur content, hardness, metals content, electrical resistivity and coefficient of thermal expansion, determine the quality and value of needle coke. Although each of these factors is important, the primary measure of product quality is the linear coefficient of thermal expansion, or CTE. The value of this measurement, in order for the product to be designated a premium coke, is not precise, but it is generally considered that a CTE of less than about 5.0×10−7/° C. (measured in the temperature range of 0 to 50° C.) is sufficient to designate the product as premium coke. However, the lower the CTE, the better, and in some cases, a batch of product having particularly low CTE may be useful in blending product to produce an overall CTE of 5.0×10−7/° C. or whatever the designated specification might be.
Hitherto, premium coke has customarily been produced in Delayed cokers from coal tar pitch and thermal tars. However, there is an insufficiently limited supply of coal tar pitch for the demand of the modern industry. Thermal tars are produced by the thermal cracking of virgin, thermally cracked, and catalytically cracked gas oils. Attempts to make premium coke from gas oil without first thermally cracking the gas oil have generally been unsuccessful. The lack of success in producing premium coke without thermally cracking the feedstock, combined with the inability to accurately identify and quantify components in coker feedstocks, has led the industry to the belief that a thermal cracking operation is needed in conjunction with a coker installation in order to produce premium coke. The trend of increased use of fluid catalytic cracking (FCC) units in place of thermal cracking units had forced the industry to search for a feedstock for delayed coking that does not require a thermal cracking step prior to coking. FCC decant oil has been found to be suitable for needle coke formation along with other aromatic rich streams. In addition to this, aromatic extract has also being used as feed constituents for production of needle coke.
U.S. Pat. No. 2,775,549 to Shea discloses an early process for making premium coke from certain petroleum residues. In U.S. Pat. No. 2,922,755 to Hackley, a process is disclosed wherein reduced crude can be mixed with thermal tar to produce a mixture which results in a premium grade coke upon carrying out the delayed coking process provided that this reduced crude is present in the weight percent range of about 10 to about 30. A process for the simultaneous manufacture of regular and premium coke is described in U.S. Pat. No. 3,472,761 to Cameron.
U.S. Pat. No. 3,759,822 describes a method for producing premium coke comprising coking a blend of a thermally or catalytically cracked heavy oil having a high aromatic content with a quantity of a pyrolysis tar obtained from the high temperature cracking of petroleum distillates to produce olefins under conventional coking conditions.
U.S. Pat. No. 4,130,475 describes a process for producing needle coke from a fresh feedstock having a specified gravity, carbon residue and boiling distribution and incorporating thermal tar, internally produced through thermal cracking of gas oil obtained from delayed coking as a supplement to the fresh feed. Japanese Pat. No. 58025385 describes a process for production of needle coke using the extract of the thermally cracked residue of heavy petroleum oil where extraction is carried out using a cracked distillate as solvent and the solvent is not recovered from the extract.
U.S. Pat. No. 4,466,883 describes a process for the production of an improved grade of needle coke using a feedstock comprising selected proportions of a pyrolysis furnace oil and hydro-desulfurized blend of a clarified oil and a lubricating oil extract employing the steps of heat soaking in presence of 20-200 ppm of sulfur, flashing to separate the pitch as residue, fractionation of the flashed oil to afford a cokable bottom fraction and subjecting the cokable bottom fraction to delayed coking.
U.S. Pat. No. 4,075,084 discloses a process of producing needle coke from specific aromatic mineral oil feedstocks through fractionation and hyrdofining of the major lighter fraction and subsequent blending of the heavier fractions obtained thereof.
As per U.S. Pat. No. 4,108,798, a highly crystalline coke can be prepared by heat-soaking a petroleum feedstock which is selected from the group consisting of virgin crude oil, distillation residues, cracked residues and hydro-desulfurised distillation and cracked residues in the presence of 30-200 ppm added dissolved sulfur at a temperature more than 230° C., separating non-crystalline substances as pitch, recovering a heavy cokable residue from the pitch free feed, and subjecting the residue to delayed coking.
In coking processes described in the previously mentioned patents, as well as other variations of the basic coking process, feedstocks normally comprise blend of residual oil viz., thermal cracker residue, FCC decant oil, extract, etc., which have been obtained from various processing steps prior to introduction to the coker. The properties of these feedstocks prior to coking is subject to variation even when they have been sourced from similar processing units due to the likely variation and upsets in the operation. In many cases, for unexplained reasons, product quality has failed to meet specifications even though the feedstock was from the same origin as earlier feedstocks, which produced high quality product.
Moreover, worldwide, there are refineries, especially those of smaller capacity, that do not have any thermal cracker for gas oil feedstock or FCC or an extraction unit. As per the prior art of the needle coke production as disclosed by the patents, these refineries would not be able to produce the needle coke since, in general, thermal cracker residue, FCC decant oil and extract have been used at different proportions to make the needle coke feedstock. It was our endeavor to discover a process whereby without having the units of thermal cracker, FCC or extraction unit, production of needle coke is possible with minimum variation in coke quality.
In this line, a process for producing both non-crystalline and high-crystalline petroleum cokes directly from a virgin crude oil having a sulfur content of lower than 0.4% by weight by a two-stage process is disclosed in the U.S. Pat. No. 3,959,115 by Hayashi et al. In this process, virgin crude oil is preheated in a tube heater to a temperature of 460-520° C. under a pressure of 5-20 kg/cm2, and then subjected flashing into a coking drum at a temperature of 410° C.-430° C. under a pressure of 2-10 kg/cm2 to produce a non-crystalline coke. The heavy residue thus obtained undergoes second stage coking to produce crystalline coke. The process uses virgin crude oil as feed to avoid deposition of coke on the inner wall of heater tubes at the conditions of temperature and time of the heat soaking step. The yield of reduced crude oil (RCO) is in general in the range of 30 to 50 wt % of virgin crude depending on the type and source. Therefore, when the entire crude is considered for delayed coking, the quality of distillate products are inferior in terms of more olefins in gasoline and more aromatics and olefins in kerosene and gas oil range products as compared to those obtained from atmospheric distillation process. Also, the operating expenditure for such a process is expected to be much higher. Therefore, in the era of stringent product quality, the process for production of needle coke using crude oil as feed has several obvious disadvantages.
A variant of the above-mentioned process by the same author as disclosed in U.S. Pat. No. 4,049,538 proposes a process for the production of high crystalline coke by adopting a step of heating and soaking the feedstock in a tube heater in the presence of basic compounds, introducing the heated feedstock to a flashing column for removing non-crystalline substances as pitch, fractionating the distillate from the fractionating column to obtain a heavy residue which is subjected to delayed coking to form a high crystalline coke. In this process, the complete removal of non-crystalline material is difficult since it is based on separation in the flashing column. If the cutting in the flashing column is deep to assure the removal of non-crystalline substances, this will result in lowering in both the yield and quality of coke obtained in the coking stage. Moreover, the alkali metal compound used in the heat soaking step will end up in the pitch, the disposal of which is highly difficult.
A similar process is disclosed in U.S. Pat. No. 3,617,480 which describes a process for producing high quality petroleum coke through segregation of a high boiling gas oil fraction from the products of the coking of a petroleum residuum and subsequently processing the same heavy oil fraction under delayed coking conditions to produce superior quality petroleum coke. In this process, it is very difficult to achieve the quality of the crystalline coke from different types of petroleum residues from varying sources since there is no inbuilt mechanism to monitor the quality of feed for the second stage coking which is reflected in considerably high coefficient of thermal expansion, a critical parameter that determines the quality of needle coke. In our opinion, the coke produced from heavy gas oil type of stream cannot meet the stringent CTE specification of today's premium needle coke.
According to the present invention, we provide a process for producing high crystalline needle coke suitable for manufacturing graphite electrodes using heavy atmospheric distillation residues having sulfur no more than 0.7 wt % which is in general not suitable for direct production of needle coke having CTE of less than 1.1×10−6/° C. by any methods described in the prior art processes. The process of this invention employs two stages or two modes of delayed coking scheme of different reaction severities wherein the reaction severity in the first stage or mode facilitates the formation of at least two fractions of predetermined characteristics and the second stage or mode favors the formation of mesophase crystalline structure in the coke. When employed in a preferred sequence of process steps, the process of the present invention affords a needle coke having superior physical properties, including a very low co-efficient of thermal expansion.